With age, there is gradual decline in muscle regenerative potential, resulting in incomplete regeneration and fibrosis. Studies from our laboratory have shown that the age-dependent decline in regenerative potential is due to a loss of functionality of the resident muscle stem cells known as "satellite cells" (SCs). The major goals of the studies of this proposal are to explore molecular pathways implicated in aging of SCs and to understand the mechanisms underiying the age-related decline in SC functionality. Our published and ongoing studies have focused on the role of Wnt signaling in the age-related suppression of SC function, at least in part by inducing a conversion of myogenic stem cells to a fibrogenic lineage. As such, the Specific Aims of this proposal are: 1) to analyze the mechanisms by which Wnt signaling and transcriptional readouts lead to age-related suppression of SC functionality;2) to analyze how regulators of the Wnt pathway account for specific transcriptional responses and changes with age;and 3) to study the epigenetic profiles and determinants of young and old SC function. We will examine transcriptional programs and regulators in adult and aged SCs, testing for changes in cellular responses with age. A recent area of interest is the role of the co-activator of the Wnt pathway, BCL9, in SC aging and in determining the transcriptional response to Wnt by acting as a "histone decoder". Therefore, a major focus of these studies will be the characterization and regulation of the epigenetic status of adult and aged SCs and the regulation of both the Wnt pathway and the epigenetic state of aged SCs by BCL9. Using ultra-high-throughput sequencing, we will examine transcription factor targets in the Wnt/p-catenin pathway, interactions between this pathway and signaling via TERT (in collaboration with Project 2) and Foxo3 (in collaboration with Project 3), and global epigenetic profiles of adult and aged SCs. We will directly examine SC aging in cohorts of mice in which BCL9 is genetically deleted in the SC compartment. These studies will form the basis of a comprehensive analysis of the molecular basis of age-related functional changes of a stem cell population and how those changes both contribute to and are regulated by aging of the tissue and organism. RELEVANCE (See instructions): This Project will investigate basic mechanisms of aging of stem cells in skeletal muscle, a process which renders them less able to participate in muscle maintenance and repair after injury. Our research focuses on biochemical pathways that we believe can be modulated to enhance the functional properties of aged muscle stem cells, thereby reducing muscle atrophy and improving muscle repair after injury or disuse.